The present invention relates to a waveform display apparatus used for displaying a waveform of a video signal, especially to a waveform display related to a phase offset between external and internal reference modes.
Television broadcasting is conducted by preparing programs using video signals from video signal sources. The same program is sometimes broadcast using HDTV and simultaneously using SDTV such as NTSC. A station uses a video signal as a synch reference to synchronize the video signals of the different formats or from different video signal sources. The reference video signal is typically a black burst (BB) signal of the NTSC format.
A waveform monitor is suitable for watching and measuring various conditions of the video signals by displaying waveforms of the video signals provided at the video inputs. The waveform monitor has an internal reference mode and an external reference mode. The internal reference mode determines waveform display timing of an input video signal (a video signal under test) by deriving a trigger signal from the input video signal itself. The waveform monitor is previously adjusted not to cause a phase difference between the video signal and the trigger signal so that if a 42nd line is designated by line select function a waveform including the 42nd line is displayed. It is preferable to confirm the condition of the video signal under test.
The external reference mode derives the trigger signal from a video signal (an external reference video signal) provided at the external reference input terminal and a waveform of a video signal under test is displayed according to the trigger signal. In this case, there may be a phase difference between the video signal under test and the trigger signal so that even if a 42nd line is designated by line select function a displayed waveform may not include the 42nd line. Therefore it is suitable for watching and/or measuring an advance or delay (phase relationship) of the video signal under test relative to the external reference video signal. Typically, it is effective that the BB signal is provided at the external reference input to confirm a phase relationship of the video signal under test relative to the BB signal.
For convenience of the explanation below, peripheral technologies related to the waveform display apparatus are described here. Waveform display timing of a video signal under test in the external reference mode provides a stable waveform display of a video signal under test by simply deriving a trigger signal from the vertical and horizontal synch signals of an external reference video signal if the external reference video signal and the video signal under test have the same formats. However, if the field frequencies of the external reference video signal and the video signal under test are the same but the line frequencies are different a waveform display position would not be stable even by deriving a trigger signal from the horizontal synch signal of the external reference video signal because the line frequency is different from that of the video signal under test. U.S. Pat. No. 6,130,708, which is hereby incorporated herein by reference, discloses an invention that solves this problem and it is widely used in products, such as the WFM700 waveform monitor, WVR7100 waveform rasterizer, manufactured by Tektronix, Inc., Beaverton, Oreg. The invention produces a trigger signal of which phase becomes zero at any of the vertical synch signals of the external reference video signal and has the same line frequency as the video signal under test.
The WVR7100 waveform rasterizer features a function called timing display. FIG. 1 is an example of the timing display 10 concerning an external reference video signal and a video signal under test that have different frame frequencies. It measures how many lines and how many micro seconds the video signal under test advances or delays relative to the external reference video signal, and then the measured values are indicated in fields 20 and 22 respectively and timing relationship between both the signals is graphically displayed. U.S. Pat. No. 6,975,349 and U.S. Patent Publication No. 2004/0239764, which are both incorporated herein by reference, disclose it. The measurement is realized with a counter that counts a reference clock for a period between time points that the respective first lines of the first fields of the external reference video signal and the video signal under test arrive. The reference clock can be recovered from the external reference video signal or the video signal under test if one of them is a digital format.
At a broadcast station, the condition of a video signal, synch of the video signal relative to a reference video signal (BB signal), as well as other parameters are confirmed repeatedly during a broadcast. In doing so, the internal and external reference modes are sometimes repetitively switched repeatedly as well. At this time, it is sometime required that an initial phase relationship between the external reference video signal and the video signal under test is set up as a user desires, and then a waveform of the video signal under test is displayed to confirm whether the phase difference between them is getting larger with time.
In this case, the phases of the external reference video signal and the video signal under test are usually different at the inputs of the waveform apparatus since the paths that they have passed through are different. Therefore, if a waveform display is displayed in the internal reference mode and then the mode is switched to the external reference mode, the waveform display has a phase offset. For example, the 42nd line is designated and displayed using the line select in the internal reference mode and then the mode is changed to the external reference mode, another line may be displayed even if the same 42nd line is designated. This offset is inconvenient for observing the waveform so that a conventional video system may use an optional line that adds an offset to the external reference video signal from the synch signal generator to cancel the offset of the waveform display apparatus according to need.
Therefore it is desired to solve the phase offset of the video signal waveform display when the internal and the external reference modes are switched for improving operability.